1. Field of the Invention
The invention relates to a method of fabricating a thin film transistor array.
2. Description of the Related Art
One of conventional methods of fabricating a thin film transistor (hereinafter, "thin film transistor" is referred to simply as "TFT") array to be used for a liquid crystal display (LCD) has been suggested in Japanese Unexamined Patent Publication No. 5-243333. The suggested method is explained hereinbelow with reference to FIGS. 1, 2, 3A and 3B.
As illustrated in FIG. 2, a gate electrode 31 and a gate bus line 32 connecting to the gate electrode 31 are first formed on a transparent, insulating substrate 30. The gate electrode 31 and the gate bus line 32 are made of metal such as chromium. Then, as illustrated in FIG. 3B, operative semiconductor films 35 are successively formed, and as a result, operative semiconductor islands are formed on the gate electrode 31. Each of the operative semiconductor films 35 is constituted of a multi-layered gate insulating film 33 including layers made of insulators such as silicon dioxide and silicon nitride, and an amorphous silicon film 34.
Then, there are formed contact holes 38 throughout the gate insulating film 33 for electrically connecting a lower metal layer 36 to an upper metal layer 37. Then, there are formed the upper metal layer 37, a signal line 39, a source electrode 40 and a drain electrode 41. For instance, they are made of chromium.
Then a transparent electrode 42 and a pixel electrode 43 made of indium tin oxide (ITO) are formed at a terminal section. Subsequently, phosphorus-doped amorphous silicon between the source electrode 40 and the drain electrode 41 is removed. Thus, there is completed TFT.
Then, a protection film made of silicon nitride is deposited all over the substrate, followed by removal of portions of the protection film located above a gate terminal, a drain terminal, and the pixel electrode. Thus, a TFT array substrate is completed.
For the purpose of shortening photolithography steps, in particular, of removing a portion of an insulating film at a terminal, various methods have been suggested in Japanese Unexamined Patent Publications 62-298117, 62-298118 and 6-102528.
The above-mentioned Japanese Unexamined Patent Publication 62-298117 has suggested a method where a film for protecting TFT is formed without removing a photoresist film having been formed when an upper metal layer was formed, and a photolithography step for removing the protection film by lift-off is omitted.
The above-mentioned Japanese Unexamined Patent Publication 62-298118 has suggested a method where one photo-mask can be omitted by leaving a metal film not removed on ends of a gate bus line, a drain bus line, and an auxiliary capacitive bus line, and exposing a substrate to a light from its rear with a negative type photoresist being deposited on the substrate.
The above-mentioned Japanese Unexamined Patent Publication 6-102528 has suggested two methods. One of them provides an advantage of omitting one photolithography step by the steps of forming a protection film over a substrate after formation of an upper metal film, forming a transparent electrode without removing a photoresist film used for forming holes throughout a gate bus line end, an auxiliary capacitive bus line end, and a drain bus line, and then forming a pixel electrode and an electrode covering a terminal by lift-off.
The other provides an advantage of omitting one photolithography step for forming a contact hole electrically connecting an upper metal layer to a lower metal layer, by the steps of forming a polyimide film with ends of a gate bus line and an auxiliary capacitive bus line, forming a gate insulating film and an operative semiconductor film, forming a drain electrode, a pixel electrode, and a protection film, forming a photoresist film having a pattern by which holes are formed at ends of a drain bus line, a gate bus line, and an auxiliary bus line, etching the end of the drain bus line with both a transparent electrode located above the drain bus line end and the end of the gate bus line serving as an etching stopper, and etching the end of the auxiliary capacitive bus line with the polyimide film serving as an etching stopper, and removing the polyimide film by dry etching.
However, the conventional methods of fabricating a TFT array substrate such as the method suggested in Japanese Unexamined Patent Publication No. 5-243333 have a problem that two photolithography steps have to be carried out, one of which is for forming a contact hole for electrically connecting upper and lower metal layers with each other, and the other is for removing a protection film located above an end of a bus line.
In accordance with the methods having been suggested in Japanese Unexamined Patent Publications Nos. 62-298117 and 6-102528, a gate insulating film is not formed at an end of a bus line. Though the methods are useful for fabricating a single kind of TFT array substrate, but not suitable to fabrication of a plurality of kinds of TFT array substrates, because a region where a gate insulating film is to be formed has to be changed for each of kinds of TFT array substrates.
In addition, since the methods having been suggested in Japanese Unexamined Patent Publications Nos. 62-298117 and 6-102528 employs the lift-off process by which, when a photoresist film is removed, a film formed on the photoresist film is also removed together with the photoresist film, there generates debris when the film and the photoresist film are removed, resulting in that a fabrication yield is lowered.
The conventional method having been suggested in Japanese Unexamined Patent Publication No. 6-102528 additionally has a step of forming a polyimide film at an end of a bus line of a lower metal layer, and as a result, has an extra photolithography step.
The conventional method having been suggested in Japanese Unexamined Patent Publication No. 62-298118 makes it possible to reduce a photo-mask using step, but fabrication steps of the method remains the same as prior methods, resulting in that a fabrication cost would not be reduced.